In circuits having two input ports, the input signal can be divided into the sum of a common mode signal and a differential mode signal. A common mode choke is a circuit that blocks passage of the common mode component of an input signal. A typical existing common mode choke is illustrated in FIG. 1. It consists of a pair of wires 11 and 12 wound onto a ring 13 of ferromagnetic material. Wire ends 14 and 15 serve as a pair of input ports and ends 16 and 17 serve as a pair of output ports. At input ports 14 and 15 are applied input voltage V.sub.1 and V.sub.2, respectively. The common mode component of this signal is equal to (V.sub.1 +V.sub.2)/2 and the differential mode signal is equal to (V.sub.1 -V.sub.2)/2. The windings of the wire about ring 13 produces a self inductance L.sub.1 in wire 11, a self inductance L.sub.2 in wire 12 and a mutual inductance M between these two wires. For a current I.sub.1 in wire 11 and I.sub.2 in wire 12, the voltages and currents satisfy the relationships: ##EQU1## When L.sub.1, L.sub.2 and M are equal, the mutual impedances counter the self inductances to eliminate the common mode component at the output ports 16 and 17. This can be seen by rewriting equations (1) and (2) in terms of common mode and differential mode components .DELTA.V.sub.c .ident.(.DELTA.V.sub.1 +.DELTA.V.sub.2)/2, .DELTA.V.sub.d .ident.(.DELTA.V.sub.1 -.DELTA.V.sub.2)/2, I.sub.c .ident.(I.sub.1 +I.sub.2)/2 and I.sub.d .ident.(I.sub.1 -I.sub.2)/2: ##EQU2## Thus, for L.sub.1 =L.sub.2 .ident.L, .DELTA.V.sub.c is proportional to I.sub.c and .DELTA.V.sub.d is proportional to I.sub.d. For L+M much smaller than L, the impedance experienced by the common mode component is much larger than the impedance experienced by the differential mode component. Such large impedance experienced by the common mode component discriminates against this component. To the extent that the coupling coefficient K.ident.-M/L is less than one, a common mode component will appear in the output. The sign of M can be reversed by reversing the direction that either of wires 11 and 12 is wound about ring 13, thereby converting this common mode choke into a differential mode choke.
When input port 15 of the common mode choke is grounded, the output voltages V.sub.3 and V.sub.4 on output ports 16 and 17 are opposite in sign and are equal in magnitude to one half of V.sub.1. This version therefore functions as a splitter. When output port 17 is grounded, the output voltage V.sub.3 is equal to V.sub.1 -V.sub.2. This version therefore functions as a combiner.
Unfortunately, the choke of FIG. 1 does not function effectively at high frequencies. In general, ferrite materials have permeabilities which fall off rapidly at frequencies above several megahertz. At frequencies on the order of 1 GHz or more, the small wavelength (on the order of or less than 4 inches) of the signals becomes comparable in the size to the discrete components of the common mode choke of FIG. 1, thereby enabling resonant effects to be important. For such small wavelengths, variations in spacing between windings and other components of that choke can produce resonant effects that result in large variations in operating characteristics, thereby making these devices unsuitable for use at such high frequencies.
In the article C. Norman Winningstadt, Nanosecond Pulse Transformers , IRE Trans. Nuclear Science, vol. NS-6, pp. 26-31, March 1959, a transformer is presented that utilizes distributed rather than lumped elements. As discussed in the article Richard E. Matick, Transmission Line Pulse Transformers--Theory and Applications, Proceedings of the IEEE., Vol. 56, No. 1, January 1968, pp. 47-62, the effects of unwanted "stray inductance and capacitance, if uniformly distributed, can be absorbed into the characteristic impedance of the transmission line, thus avoiding resonant points and providing a broadband device". This article analyzes the transmission of pulses in short (i.e., comparable in length to a pulse) and long transmission lines above a ground plane and applies this teaching to baluns and transmission line pulse transformers.